text: Nuclear weapons employ high quality, highly enriched fuel exceeding the critical size and geometry (critical mass) necessary in order to obtain an explosive chain reaction. The fuel for energy purposes, such as in a nuclear fission reactor, is very different, usually consisting of a low-enriched oxide material (e.g. UO). There are two primary isotopes used for fission reactions inside of nuclear reactors. The first and most common is uranium-235. This is the fissile isotope of uranium and it makes up approximately 0.7% of all naturally occurring uranium. Because of the small amount of U that exists, it is considered a non-renewable energy source despite being found in rock formations around the world. Uranium-235 cannot be used as fuel in its base form for energy production. It must undergo a process known as refinement to produce the compound UO or uranium dioxide. The uranium dioxide is then pressed and formed into ceramic pellets, which can subsequently be placed into fuel rods. This is when the compound uranium dioxide can be used for nuclear power production. The second most common isotope used in nuclear fission is plutonium-239. This is due to its ability to become fissile with slow neutron interaction. This isotope is formed inside nuclear reactors through exposing U to the neutrons released during fission. As a result of neutron capture, uranium-239 is produced, which undergoes two beta decays to become plutonium-239. Plutonium once occurred as a primordial element in the earth's crust, but only trace amounts remain, so it is predominantly synthetic. Another proposed fuel for nuclear reactors, which however plays no commercial role as of 2021, is uranium-233, which is "bred" by neutron capture and subsequent beta decays from natural thorium, which is almost 100% composed of the isotope thorium-232. This is called the thorium fuel cycle.